Interpretive Summary: This study analyzed a worldwide collection of isolates of Pyrenophora tritici-repentis, an economically important fungus that causes tan spot of wheat. Isolates of multiple races were collected from North America, South America, and Europe over a span of 1969 to 2002. Results showed that this fungal population is preferentially out crossing locally in nature and that the introduction of this population is either relatively recent or that there has been a constant worldwide flow of this fungus possibly by seed movement between continents.

Technical Abstract:
The work presented here is the first major study to analyze the genetic diversity within the worldwide population of the economically important wheat pathogen Pyrenophora tritici-repentis. The genetic structure of field populations of P. tritici-repentis was determined using AFLP markers along with sequence data from the internal transcribed spacer (ITS) region of the ribosomal DNA. Ninety seven fungal isolates were collected from naturally infected wheat and wild grass species. The collection of 97 P. tritici-repentis isolates includes races 1, 2, 3, 4, 5, 7 and 8 and was collected from North America, South America and Europe. Results show no genetic grouping of fungal races nor do results indicate grouping based on geographic location indicating the population is preferentially out crossing locally in nature and that the introduction of this population is either relatively recent or that there has been a constant worldwide flow of this fungus possibly by seed movement between continents.